This invention relates in general to polyimide resin foam materials, and, more specifically, to compositions and methods for foaming and fully curing polyimide resins entirely by the application of microwave energy to produce resilient, flame resistant foams.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839 disclosed and claimed certain polyimide compositions which are flame resistant and useful as coatings and adhesives.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxoimine. The ratio of oxoimine to dianhydride is preferably in the 2.3:1 to 2.7:1 range and the imidization reaction is preferably conducted at a temperature of 170.degree.-200.degree. C. for 20-60 minutes.
The polyimide forming material is then prepared by dissolving the bisimides in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution is coated onto a surface and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. The following is exemplary of the exchange reaction which occurs: ##STR1## where n is a positive integer.
The resulting coating is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not suitable for use in applications requiring a cellular or foam material, since conventional agitation foaming and addition of known blowing agents add to process costs and complexity and are not entirely effective at the relatively high polymerization temperature required.
A number of other methods and compositions have been described for the production of foams from polyimide resins, either spontaneously upon heating or by the inclusion of a foaming agent. Typical are those detailed in U.S. Pat. Nos. 3,554,939, 4,296,208, 4,315,077, 4,394,464, 4,369,261 and 4,335,120. The methods described in these patents produce good foams, but use complex, energy inefficient, heating and foaming methods and do not contemplate the use of microwave energy to accomplish foaming and curing. Another U.S. Pat., No. 4,305,796, does discuss using microwave energy to cause a polyimide resin to foam and partially cure. As pointed out in that patent, to achieve best cure, the resin should either be thermally preheated, or the microwave cavity should be thermally heated during microwave energy application. As is detailed below, the method used in this patent does not produce a fully acceptable cure by application of only microwave energy.
Thus, there is a continuing need for improved compositions and methods for producing improved polyimide foams, especially for the foaming and curing of polyimide foams by only the application of microwave energy for improved process simplicity and energy efficiency.